Optical thermometry based on luminescence intensity ratio (LIR) is a hot topic in temperature sensing, due to fast response, high precision, the benefit of non-contact, etc. In this paper, the temperature-sensing capability of GdPO 4 :1%Dy 3+ phosphor was investigated through the approach of fluorescence intensity ratio based on the thermally coupled energy levels of Dy 3+ . The results show that the intensity of the emission, recorded under a 355 nm laser excitation, markedly increases with increasing temperature from 290 to 539 K. The LIR between the intensities of the emission of thermally coupled energy levels corresponding to 4 I 15/2 → 6 H 15/2 and 4 F 9/2 → 6 H 15/2 transitions monotonously increased with increasing temperature within the investigated temperature range. The maximum value of the relative sensitivity reaches the value of 1.55% K −1 at 290 K, which is higher than the values reported in previous studies for similar phosphors. This finding suggests that the produced Dy 3+ -doped GdPO 4 phosphor is a promising candidate material for application in optical temperature sensing. Furthermore, the high relative sensitivity of 1.34% K −1 was attained in the physiological temperature of the human body (310 K), which qualifies the produced phosphor for biological applications.